Alabama tornadoes: La Nina may bring more storms (slideshow)

Alabama could be in for another season of strong storms, thanks to an unusual recurrence of weather disruption known as La Nina.

A La Nina occurs when the ocean surface temperatures off the coast of Peru are colder than normal. That sets off a series of changes to weather patterns around the globe. Meteorologists believe that La Nina conditions in 2011 helped set the stage for tornadoes last spring.

Alabama state climatologist John Christy said the strong La Nina in 2010 and 2011 led meteorologists to predict that La Nina conditions would return after fading in the summer of 2011, and indeed they did. The double dip La Nina is uncommon.

"It is fairly rare but not unprecedented," Christy said.

During a La Nina, it tends to be warmer than normal in the southeastern United States. In north Alabama, it tends to be wetter. In a normal year, storms are randomly distributed across the country, but in a La Nina year, the storms tend to occur more frequently in the Ohio Valley. The southern trailing edges of those storms sweep across north Alabama, Christy said.

In south Alabama, La Nina is associated with drought. In Florida and the southern portions of Alabama and Georgia, extremely dry conditions are in place and have been since the beginning of the La Nina in 2010.

Birmingham and areas of central Alabama roughly corresponding to the east-west path of Interstate 20 lie in a transition zone between the two.

One difference between this year's mega-weather patterns and last year's is the behavior of the Arctic Oscillation.

When the Arctic Oscillation is positive, as it has tended to be this winter, cold arctic air tends to stay penned up in high northern latitudes.

When the Arctic Oscillation is negative, as it tended to be in 2011, it allows more cold air to surge into the U.S. The sharper contrast between cold air from the north and warm air from the Gulf strengthens wind shear, which tends to favor tornado formation.